1. Field of the Invention
The present invention relates to an imaging device, and in particular, it relates to a solid-state imaging device in which a plurality of face plate elements of an imager are integrated on a semiconductor substrate.
2. Description of the Related Art
A solid-state imaging device needs to have the same resolving power as a pick-up tube employed for a television broadcast. Therefore, a matrix of picture elements (photoelectric conversion elements) arranged in 500 units in the vertical (column) direction and in 800 to 1000 units in the horizontal (row) direction on a semiconductor substrate, and scanning devices corresponding thereto, are necessitated. Accordingly, the aforesaid solid-state imaging device is manufactured by using MOS LSI technology enabling the realization of high integration, and a charge coupled device (hereinafter called CCD), a MOS transfer or the like is used generally as a constituent element thereof.
These prior arts will be described hereunder with reference to drawings.
FIG. 1 shows a circuit diagram of a conventional CCD type solid-state imaging device. In this figure, an example of picture elements in assumed numbers of 2.times.2 is cited for simplicity. In the present drawing, numerals 11, 21, 31 and 41 denote photoelectric conversion elements (photodiodes) for converting incident light into an electric charge, respectively, 6, 7, and 8 denote CCD for signal charge transfer, 901 and 902 denote driver transistors for a source follower, and 903 and 904 denote load transistors for the source follower. Besides, 501, 502 and 503 denote current buffer circuits, 504, 505 and 506 resistances, 507 and 508 capacitances, 509 and 510 switches, and 511 a power source. Components 501 to 511 form a correlated double sampling circuit 500. When light falls on the photodiodes 11, 21, 31 and 41, a signal charge corresponding to the incident light is accumulated in each photodiode. Signal charges thus accumulated are transferred sequentially to the gate of the source-follower driver transistor 901 by CCD 6, 7, and 8. An output of the source-follower driver transistor 902 is inputted to the correlated double sampling circuit 500. Moreover, the correlated double sampling circuit 500 delivers a difference between the outputs of the source-follower driver transistor 902 delivered before and after the signal charge is given thereto. In some detail, the correlated double sampling circuit 500 inputs beforehand to the capacitance 507 the output of the source-follower drive transistor 902 delivered before the signal charge is given, in the state in which the switch 509 is ON and 510 OFF. Next, in the state in which the switch 509 is OFF and 510 ON, the circuit takes out in the capacity 508 the output of the source-follower driver transistor 902 delivered after the signal charge is given. Thereby the difference between the outputs delivered before and after the signal charge is given is accumulated in the capacity 508. The device of this kind is discussed in the IEEE JOUNRAL OF SOLID-STATE CIRCUITS, Vol. SC-9, No. 1, February 1974, pp. 1 to 12.
Next, a description will be made on a MOS type solid-state imaging device with reference to FIG. 2. The present figure shows an example in which the number of picture elements is assumed to be 1 for simplicity. Numeral 11 denotes a photodiode operating like the one in FIG. 1, 601 a transistor for signal amplification, 604 a switch, 605 a load resistance, and 606 a power source. The components, a switch 602 and a power source 603, form a reset circuit. Numeral 4 denotes a signal line. When light is incident on the photodiode 11, a signal charge corresponding to the incident light is generated in the photodiode 11. This signal charge is amplified to be an electric current by the transistor 601. This current is transferred through the signal line 4 and outputted to an output end.
In the prior-art example of FIG. 1, the signal charge is transferred as an electric charge to an output amplifier 9 by using CCD 6, 7 and 8. Therefore noise mixes in the charge while it is transferred by CCD 6, 7 and 8, and this causes a problem in that S/N tends to be deteriorated by the noise charge.
Particularly, it happens that a part of incident light leaking in through a gap of a photo-shield film causes noise charge, and this charge leaks into CCD 6, 7 and 8, thus causing a large problem of a smear phenomenon.
In the prior-art example of FIG. 2, on the other hand, the nonuniformity in an output signal level, which is called an offset and caused by the nonuniformity in impurity concentration or an interface level under the gate of the charge-amplifying transistor 601 in a plurality, is outputted as it is. This nonuniformity in a gain is observed as if it were a signal, and consequently produces a proble of noise called fixed pattern noise being generated.